1. Field of the Invention
The present invention relates to a communication line composed by one or more signal transmission element, where each element is swapped with another wire at one or more points of its line.
This application is based on patent application No.Hei 10-028833 filed in Japan, the content of which is incorporated herein by reference.
2. Description of Related Art
A pair of conventional signal lines generally comprises a pair of twisted wires made by twisting two conductors in order to prevent noise and crosstalk by the mutual interference of the pair of wires. There is the problem in the transmission of the two differential signals with the conventional twisted wires in that the two differential signals have a relative phase shift caused by the delay of signals.
The insulators used as covers for each of the twisted wires have respectively different physical characteristics since they are manufactured with the mixture of the various dyes and pigments, and also they have respectively different dielectric constants as they have different foaming degrees and they have twisted deformations. Consequently, there is a difference in the signal propagation times between the two twisted wires, which is called a differential skew.
In the process of manufacturing the two twisted wires, the tensions added to the two wires are sometimes unequal. In these cases, one wire to which is added stronger tension is twisted shorter than the other wire to which is added the weaker tension, and there is a result that the two wires to be twisted have different lengths.
Also, as an example of the parallel transmission of more than two independent signals, the parallel transmission of the four signals using the UTP (Un-shielded Twisted Pair) cable (specifically 4 pairs of twisted wires) which is currently used in the field of LANs (Local Area Network) is discussed below. This UTP cable is generally intended to give each pair of its wires a different twisting pitch in order to decrease the crosstalk. By this and the above reasons, because of the different conductors and different tensions, four pairs of wires have respectively a different length, and consequently have different propagation times for the four signals from the input terminal to the output terminal of the UTP cable.
The more detailed problem relating to the twisted wires will be discussed below in the two cases: differential transmission and parallel transmission.
(1) Differential Transmission
The differential skew, which occurres during manufacturing, is accumulated almost constantly toward the longitudinal direction of the wire. The increasing ratio of the accumulation depends on the respective manufacturing apparatus. This differential skew causes a distortion of the signal waveform, and the influences are described below in the two sections (1-a) and (1-b).
(1-a) Influence by the Phase Shift at the Output Terminal
In FIG. 2, a signal is separated into differential signals A and B by the differential amplifier 22, and the signals A and B are transmitted through the conductors 20 and 21 which are respectively insulated, and output signals C and D are gathered together into one signal by the differential amplifier 23. Signals A and B are transmitted through the channel which is made between the conductors 20 and 21, and the insulator of the cable or the earth. In the case shown in FIG. 2, the conductor 20 through which the signal A propagates has a longer propagation delay than the conductor 21 through which the signal B propagates. Consequently, the output signal C has a phase delay in comparison with the signal D. When the phase delay becomes bigger than the time of 1 bit of the original signal, the receiver circuit can not receive the signal correctly and it results a bit error. In FIG. 2, the pulse of the conductor 21 in the middle of the line shows that the conductor 20 and 21 are twisted.
(1-b) Influence of the Waveform Distortion by the Electromagnetic Induction
In FIG. 3, signals E and F which passed the differential amplifier 32 are transmitted through the conductors 30 and 31, and output signals G and H are gathered together into one signal by the differential amplifier 33. Here, it is supposed that the conductor 30 in which the signal E propagates has a bigger propagation delay than the conductor 31 in which the signal F runs.
In FIG. 3, a phenomenon caused by the differential skew is arisen wherein a signal with an early phase is decreased its amplitude, and a signal with a late phase is increased its amplitude. This is because of the electromagnetic induction by the phase shift of the differential waveform which is transmitted in the conductor. Additionally, if the phase shift increases, the distortion increases; and if the phase is inverted, the distortion in decreased. Accordingly, when the length of the line which has an early phase and that which has a late phase are not balanced, the output waveform is unbalanced and loses its differential voltage symmetry because the distortion of the waveform is not compensated to the end of the line. This distortion of the waveform also increases not only the decrease of the amplitude but also the jitters.
The degradation of the waveform makes it difficult to recognize the signal correctly at the receiving circuit, and it also causes bit errors and prevents high speed and long distance communication.
As an example of the requirement for the difference in the propagation delay time of the twisted wires, 1000 Base-CX being discussed in IEEE P802.3z provides a difference of propagation delay time less than 150 ps. This means that the allowance of generally used twisted wires is approximately 4 cm. This accuracy is actually impossible to realize.
(2) Parallel Transmission
An example of the parallel transmission with twisted wires will be discussed below. In FIG. 4, the data is transmitted in parallel with two pairs of twisted wires. Here, serial signal I is divided into two parallel signals J and K by serial-parallel converter 42, and signals J and K are transmitted from transmission circuit 43 and 44 through two pairs of twisted wires 40 and 41 to the receiving circuit 45 and 46. After passing the receiving circuit 45 and 46, two parallel signals L and M are converted in to one serial signal N by parallel-serial converter 47. In this case, as two bit signals can be transmitted simultaneously, double the quantity of the data signals can be transmitted, compared to the single pair of twisted wire.
However, if there is a difference in the propagation delay time between each of the twisted wires, the sequence in the pair data is changed making it difficult to retrieve the data correctly. In FIG. 4, the propagation time through the twisted wires 40 is longer than that of the twisted wires 41. Accordingly, the signal through the twisted wires 40 is more delayed at the output end than the signal propagating through the twisted wires 41.
Therefore, conventional parallel transmission requires degrading the transmission speed or shortening the distance of the transmission within an allowable range of the propagation delay time between the twisted wires.
As a signal transmission element for parallel transmission, not only a balanced cable excluding the twisted wire but also an unbalanced cable such as a coaxial cable or an optical cable are satisfactory.